Recent industrial tests of amorphous alloys under realistic working environments have indicated that the wear and corrosive resistances of this new category of alloys are at least one order of magnitude higher than that of conventional alloys currently in use. Other amorphous metal compounds are of interest as superconductors and magnetically soft alloys, etc.
The formation of amorphous metals requires varying degrees of rapid cooling. Three techniques currently in use have been most successful in fabricating metallic glasses of various geometries and sizes: 1. Liquid quenching (LQ), 2. Sputtering, and 3. Electrodeposition (ED). The first preparation of an amorphous metal from the corresponding liquid was done by a gun technique. In this process, a diaphragm is ruptured by high pressure gases, the ensuing shock waves travel down the tube to a crucible with a small hole in the bottom. The molten sample is held in the crucible by its surface tension before being driven out of the hole in the form of small droplets by the shock waves. The droplets then impinge on a metal substrate, spreading out and overlapping to form an irregular foil. Other variations of this fundamental technique include twin roll technique, melt spinning, melt extraction, pendent drop process, laser glazing, chill block casting, etc. A variety of atomic deposition techniques have also been utilized to form amorphous metals. The latter techniques have higher effective cooling rates than liquid quench processes and thus present the potential for retention of phases with considerably higher free energy excess than the equilibrium phases.
In all the above-mentioned techniques, a crucible and/or substrate must be used at one point in the process. The intimate contact of the melt with a foreign surface inevitably introduces impurities into the molten metal, which become heterogeneous nucleation sites and detrimentally increase the rate of crystalline growth within the melt during its cooling process. In fact, recent experiments on PdSi have produced conclusive evidence that the extremely high rate of cooling required in the metallic glass formation is primarily due to the necessity to suppress this type of nucleation process.
Important progress in the theoretical and experimental areas has been made in recent years to provide conclusive evidences that:
1. Surface heterogeneous nucleations were responsible for activating global nucleation process;
2. Heterogeneous and homogeneous bulk nucleations played insignificant roles in an overall crystallization process; and
3. For the same cooling rate condition, by decreasing the number of surface heterogeneous nucleation sites, the size of the amorphous samples was increased.
Logically, if the surface heterogeneous nucleation sites could be reduced in number or eliminated altogether, the only crystallization process left is that due to the bulk, which could be suppressed with a very modest cooling rate. Depending on the size of the sample, the rate could be as low as 1.degree. K./sec. With a low cooling rate, the homogeneous nucleation rate may be small enough to permit bulk formation of amorphous alloys.
Some earlier attempts to form bulk amorphous alloys have employed containerless processing. In this earlier work melts were injected into a drop tube. The gaseous atmosphere was selected to minimize surface heterogeneous nucleation sites.
Theoretically, the containerless processing of molten alloys under high vacuum will certainly eliminate environmental impurities from making contact with the melt during the solidification period, thereby enhancing the conditions favorable for bulk homogeneous nucleations. In this case, the quench is due to radiative cooling. If the starting alloy is idealistically pure, this cooling rate may probably be sufficient for the formation of bulk metallic glasses. Realistically, however, this kind of condition may never be achievable in laboratory. Or it may not be economically feasible.
In addition, realistic processing time in a drop tube may never exceed several seconds. During this time period, the sample must be cooled down enough to stand the impact of landing. This may call for a cooling rate more rapid than that due to radiation alone. Consequently, some exchange gas must be used. This may expose the melt to external impurities such as O.sub.2 and H.sub.2 O.
Preliminary experiments on a PdCuSi system using a drop tube facility to produce amorphous solid spheres of several millimeters in diameter have been successful. Rapid cooling is provided by a 200 mm Hg helium exchange gas in the free-falling path of the droplet. Practical difficulties have limited the processing time of this technique to only several seconds. Space, on the other hand, provides an ideal containerless and zero-gravity environment. Many experiments along this line have been considered and proposed. A terrestrial levitation apparatus which is electrostatic, electromagnetic or acoustic in nature has also been considered and a development of such apparatus is in progress.
The electrostatic levitation apparatus has been limited to manipulate materials of low specific gravity. The electromagnetic system can levitate and heat samples of high gravities. However, the rapid quenching of the samples is not readily available. Acoustic levitation systems currently in use for terrestrial applications in the past could not handle heavy materials with acceptable lateral positional stability. In addition, depending on the thermal properties of the material, the acoustic integrity of the apparatus deteriorates rapidly as the sample is being heated to its melting temperature.